Bandgap regulator circuits for producing voltage references have been known to the designers of analog circuits. These reference circuits provide a constant voltage which is as much as possible independent of the ambient temperature at which the circuit is operated. Circuits of this kind are to be found in several systems formed from integrated circuits.
For example, a constant voltage reference is required for cellular telephone applications, and more particularly for data communication devices based on portable telephone sets. Such sets include at least two low-drop linear regulators which must be made to strict specifications, especially as relates to rejection to the power supply line.
In addition, such regulators arc required to provide a voltage reference that is stable, even when they are supplied low values of supply voltage.
FIG. 1 of the accompanying drawings shows schematically a regulator 1 as above, currently in use with latest generation cellular phones.
The regulator 1 of FIG. 1 comprises an NMOS power transistor M1 which is driven by an operational amplifier 2 being supplied from a charge pump 3.
One input of the amplifier 2 receives a voltage signal Vbg from a bandgap regulator 4, the latter being supplied via a pre-regulator circuit 5 which produces a low-drop voltage Vreg. The pre-regulator 5 is designed to provide an output voltage Vreg with good rejection to the supply line.
This design ensures good rejection to the supply line, since the power supply is itself inherently immune to noise from the supply line VBAT.
Shown schematically in FIG. 2 is one embodiment of a conventional pre-regulator circuit 5.
As can be seen in FIG. 2, the pre-regulator circuit 5 is to be supplied a voltage VBAT, from which the pre-regulated voltage Vreg is then extracted.
FIG. 3 shows schematically, by way of illustration, some embodiments of reference blocks for deriving, from the supply voltage VBAT, the voltage Vref for the pre-regulator circuit 5.
The exemplary reference blocks shown in FIG. 3 have in common the drawback of being sensitive to possible disturbance from the supply line, and consequently, are unable to provide a pre-regulated voltage Vreg that is clear of noise.
The prior art proposes an alternative solution, as shown schematically in FIG. 4. This solution provides for the bandgap regulator to be supplied through a diode-connected transistor.
The last-mentioned solution gives good results from the standpoint of rejection to the supply line, but disallows operation at very low supply voltages, because the lowest working voltage is at least 0.7V (the diode threshold voltage) higher than the voltage that can be used in the previous example incorporating a low-drop pre-regulator.